Suspension systems incorporating air bags have previously been proposed. Air bag suspensions have been used on trucks, trailers, buses, coaches and the like for many years, and they generally provide an improved ride on highway surfaces, particularly relatively smooth highway surfaces.
When used on multi-axle vehicles, or when multiple air bags are used in conjunction with single axles on vehicles, it has previously been proposed to provide air bags on each side of the vehicle with the air bags connected by a tube or other connector of relatively small cross sectional area which restricts the flow of air between the tubes to a relatively low flow rate. The tube enables the air bags to be inflated relatively equally to provide an even height for the suspension of the vehicle above the ground.
Previously proposed air bag systems, however, have been shown to experience difficulty in providing adequate vehicle suspension when the vehicle is driven over rough terrain, particularly when such suspension systems have been used in conjunction with multi-wheeled, multi-axle vehicles. For example, for a coach having a multi-axle, rear suspension system, passing over a kerb, raised road section or the like, movement of the forward set of wheels over the impediment causes a consequent movement of the rear wheels lifting the rear wheels off the ground surface. Naturally, if drive is provided to the rear wheels of the dual rear axle suspension system, the vehicle can be stranded. A similar problem can arise with trucks or other vehicles with lazy axles when travelling over uneven roads, or when traversing relatively rough terrain. This may occur, for example, with farm related vehicles such as in attempting to load livestock or handling relatively large quantities of hay, straw, farming equipment, earthmoving equipment or the like.
With previously proposed air bag suspension systems, the air bags are generally supplied with air from an air tank using a relatively small diameter, low flow rate air tube connecting the tank to the air bags. This whole purpose of the connecting tube is to enable the air bags to be inflated and deflated, to vary the height of the vehicle above the ground depending on load conditions. The low flow rate air tubes are not designed or constructed to transfer air between air bags in response to sudden changes in pressure within the air bags and they do not quickly equalise the pressure within and between the air bags. This can cause difficulties, particularly with air bag suspension systems used in multi-axle vehicles, where it can be difficult to drive onto a ramp or the like as the valve, which is set to control the vehicle height, will react to the relative movement of one axle to thereby cause air to flow into the air bags thus causing one set of wheels to be lifted off the ramp. If that set of wheels is the set of driving wheels, further progress along the ramp may be prevented.
Another difficulty encountered with previously known air bag suspension systems is known as “tramp”. When a multi-axle vehicle encounters a rough or uneven road surface, such as a railway crossing, a cattle grid or the like, the vehicle suspension is caused to oscillate (tramp) for a substantial period of time. Such oscillation may cause damage to the road's surface as well as unduly stress a fully loaded vehicle.
Attempts have been made to overcome known difficulties using air bags for vehicle suspension systems. Australian Patent No 567664 discloses an air bag suspension whereby an air tank is mounted in the vehicle chassis directly above the air bags, and short, large diameter air ducts connect each air bag to the air tank. This system has as its aim to maintain the air pressure in each air bag relatively constant irrespective of the position of the vehicle axle to thereby reduce excessive vehicle body movement. The specification also discloses the elimination of restricting flow air lines connecting the air bags to the air tank. Each side of the vehicle has its own air tank connected to the air bags on that side of the vehicle, with the air tanks optionally being connected.
However, this proposal does not solve the problems referred to above as, when a vehicle axle moves upwardly due to an irregularity in a road surface, air in the air bag and the air tank is compressed. When the irregularity in the road surface has been passed, the pressure in the air tank and air bag would force down the axle with great speed thereby forcing the vehicle tyre onto the road surface with a great impact. Because of the resilience of the vehicle pneumatic tyre, the rebound would be sufficient to again compress air in the air bag and air tank, thereby commencing a cycle of tramping. Shock absorbers or dampers are thereby required in such a system to counteract the effects of the tramping. Such shock absorbers or dampers add to the vehicle suspension costs and provide further stress points in the vehicle suspension.
Australian Patent Application No 69220/87 proposes the damping of shock loads on the suspension system by providing a secondary air tank mounted within the primary air tank and communicating through a restrictive opening. The secondary air tank and restrictive opening combine to assist in the dampening of shock loads on the suspension. However, with the speed of operation, the large volume of air in the air tank and the large size of openings between the air bags to the air tank means that no effective dampening of tramping occurs and the system is unable to cope with uneven loads and uneven terrain.
U.S. Pat. No. 3,063,732 discloses a vehicle suspension system incorporating both leaf and air spring assemblies in combination. The specification discloses the use of air bags on a dual axle suspension with front and rear air bags connected by a hollow sub-frame to serve as pneumatic reservoirs. The system is also provided with height control valves on each side of the suspension so that the air bags on each side are independently inflated in accordance with load conditions. The air suspension is used in conjunction with leaf springs on each end of each axle. However, this suspension system does not provide means for controlling tramp or otherwise damping suspension oscillations. Further, the ride height on both sides of the vehicle need to be adjusted to change the level travel height for any given load.